This invention relates to apparatus for automatic temperature control and more particularly to such apparatus for the accurate and smooth control of temperature in vacuum furnaces over a wide range of operating temperatures.
Vacuum furnaces are often employed for batch sintering processes such as the sintering of carbides. To insure high quality products, the rate at which the furnace heats up must be carefully controlled because even small temperature variations from a desired profile can seriously degrade the quality of the finished sintered product. In the past it has been difficult to control smoothly and accurately the temperature profile in vacuum furnaces over wide temperature ranges because the heat transfer characteristics vary with temperature. Specifically, in a vacuum furnace heat transfer between the furnace chamber and the load is substantially by radiation alone; convection and conduction heat transfer mechanisms are precluded by the vacuum condition. At high temperatures, heat transfer among the furnace heating elements, the load and the temperature sensor by radiation is very efficient allowing for tight temperature control. This is so because heat transfer by the radiation mechanism is proportional to the fourth power of temperature. At low temperatures, on the other hand, control problems are caused by the thermal lags present in a vacuum system. These lags result in part from changes with temperature of radiant heat transfer, heat loss, heating element resistance, heat penetration into the load and thermodynamic properties of the load.
At low temperatures these thermal lags can result in unacceptable temperature regulation, namely, an oscillation of the furnace temperature about the desired temperature set point. In particular, as soon as the set point from a programmer exceeds the furnace chamber temperature, the controller calls for full power, thereby heating the furnace at its maximum rate. Although the power to the furnace will be reduced to zero when the chamber temperature reaches the set point, the thermal inertias inherent in the system can cause the temperature to overshoot the set point substantially. The furnace eventually will cool to meet the rising set point again causing full power to be applied. A cyclic pattern is thus established which will persist until a temperature is reached for which the control parameters and power level are more nearly appropriate. And even if stabilization were achieved, variations in the process characteristics with changing low temperatures may cause the controller to lose stability.
The prior art has attempted to deal with these control difficulties at low furnace temperatures by providing essentially two sets of control laws--one set for low temperatures and another set for high temperatures. That is, one set of controller gains and a maximum power level would be selected for low temperatures and another set of gains and maximum power for high temperatures. These prior art attempted solutions, however, have not proved successful; the abrupt switchover from one set of control laws to another set could not provide the sought after smooth heat up curve at low temperatures.
It is an object of the present invention, therefore, to provide temperature control apparatus which permits smooth and accurate temperature regulation over a wide range of desired temperatures.
It is a further object to provide such apparatus which is more flexible, affords a smoother operation and is of lower cost than previously known.